The present invention relates to current driving circuits, and more particularly to a current driving circuit which can change the recording currents of the magnetic heads of a magnetic tape storage device etc. at high speed.
The recording method of a magnetic tape storage device, a magnetic disk file storage device, etc. utilizes the magnetic storage phenomenon of a ferromagnetic substance. A coil is wound on an annular core, and current is caused to flow therethrough, to generate in the core a magnetic flux conforming to the direction of the current, so that the magnetic flux leaks from a gap provided in the core and magnetizes the surface of the magnetic substance. Since the magnetic flux in the core is inverted by changing the sense of the current, the surface of the magnetic substance is magnetized in the reverse direction by the leakage flux. In this case, when the sense of the current to flow through the coil is changed, a voltage reacting against the change (hereinbelow, termed "flyback voltage") is generated. As the changing speed becomes higher, the flyback voltage increases in amplitude. The high speed operation of the current driving circuit of the magnetic head is therefore difficult. In the typical magnetic tape storage device, the width of a track is about 1 mm, the geometries of the core are large and the inductance (L) of the head is high, so that the flybck voltage becomes very great. For such a head, there has heretofore been a circuit for three terminals wherein, as shown in FIG. 1(a), a coil L.sub.1 is provided with a center tap to which a supply voltage V is connected, and switches S.sub.1 and S.sub.2 at both the ends of the coil L.sub.1 are alternately turned "on" and "off", whereby the sense of the recording current to flow through the coil L.sub.1 is changed. With this arrangement, the two switches S.sub.1 and S.sub.2 suffice as the switches for the change-over control. However, thin film heads have been developed in recent years, and it is deemed very difficult to provide the center tap in a method of manufacturing the thin film head. Accordingly, a circuit shown in FIG. 1(b) (refer to, for example, `IBM Technical Disclosure Bulletin`, Vol. 23, No. 11, April 1981, pp. 5167-5168) is employed for the thin film head. With this circuit, as stated before, the high-speed current changing is difficult when the flyback voltage becomes great. Further, when multitrack recording is also taken into consideration, two terminals (A, B) are necessary for each track as shown in FIG. 2 in spite of narrowed track pitches, so that the connection and packaging of the heads (H) and circuits become very difficult. In FIG. 1(b), current is caused to flow from the left to the right of a coil L.sub.2 by turning "on" switches S.sub.1 and S.sub.4 and turning "off" switches S.sub.2 and S.sub.3, while in inverting the direction of the current, the switches S.sub.1 and S.sub.4 need to be turned "off" and those S.sub.2 and S.sub.3 "on". Therefore, it is necessary to provide the four switches shown.
FIG. 3 is a diagram of the fundamental arrangement of the prior-art current driving circuit shown in FIG. 1(b), while FIG. 4 is an operating time chart of the circuit in FIG. 3.
In a first state (Q.sub.1, Q.sub.4 "on") illustrated in FIG. 4, transistors 3 and 6 are turned "on" by signals G.sub.1 and G.sub.4, and transistors 4 and 5 are turned "off" by signals G.sub.2 and G.sub.3, respectively. Then, the current I.sub.S of a current source 8 is drawn in the order of a terminal 9, the transistor 3, the load (head) 7 and the transistor 6 and flows to a terminal 17. On the other hand, in a second state (Q.sub.1, Q.sub.4 "off"), the current I.sub.S is drawn in the order of the terminal 9, the transistor 4, the load 7 and the transistor 5, and the current which flows through the load becomes the opposite sense. These operations are repeatedly performed according to the signals G.sub.1 -G.sub.4, to form a current I.sub.L changing in inverted fashion. With this circuit, however, the two terminals are required per load. Further, in a case where the load 7 is inductive, the flyback voltage across the load 7 (the amplitude .vertline.V.sub.F .vertline. of a potential V.sub.F in FIG. 4) becomes great in changing the sense of the current, and hence, the switching of the transistors 3, 4 becomes difficult. Herein, in order to effect the switching operations of the transistors 3, 4 at high speed without being influenced by the flyback voltage .vertline.V.sub.F .vertline., the potentials of terminals 10 and 11 need to be higher than those of terminals 14 and 15 in excess of .vertline.V.sub.F .vertline. during "on" of the transistors 3, 4 so as to prevent the transistors 6, 5 from being saturated with the flyback voltage .vertline.V.sub.F .vertline., while they need to be lower than the lowest potential of V.sub.F during "off" of the transistors 3, 4. Here, the flyback voltage .vertline.V.sub.F .vertline. becomes .vertline.V.sub.F .vertline.=5.0 V when a current of 100 mA is to be changedover through a load of 1 .mu.H at high-speed switching of 20 nsec by way of example. In case of controlling the transistors 3, 4 by driving the terminals 10, 11 with such large amplitude, enhancement in the operating speed especially during the "off" state of the transistors 3, 4 is difficult, and disadvantageously the changing speed of the load current I.sub.L is limited in this part.